Geez... I didn't know this was such a convoluted matter... It's quite simple.

Any virgin lower that is sold as such, is not classified as a pistol or a rifle, just a 'firearm'. Once you build it out, there is also no legal issue with converting that lower receiver between a rifle and pistol configuration - you just must be sure to never have the pistol upper(anything less that 16" or 14.5" w/ fixed flash hider) attached to the lower group while a stock is attached. This even goes for just the standard stock buffer tube(removing the stock from the tube does not a pistol make).

This is similar in theory to buying a PGO shotgun, such as a Mossberg 500 Cruiser. The 4473 does not list the gun as a shotgun, but as a firearm. Since it does not come with a stock attached, and is therefore not classified under federal law as a shotgun, it is not subject to the 18.5" barrel limitation. Using a Shockwave Technologies 'birdshead' style pistol grip, you can add the Mossberg 590 AOW 14" front end without needing to register under the NFA. This is because the Shockwave grip extends rearward, instead of down, making the OAL over 26"(26.5"). It's an easy and legal way to have an SBS without registering under NFA. [see here]

You can have a pistol buffer tube attached and put on your 10.5" (or whatever) size upper group... or even your 16"+ upper group... although it would be weird shooting a pistol tube with an upper that large.

Switch back and forth as much as you want... just be sure to never have it in a configuration that would qualify under the NFA.

On a somewhat related note, some people build out a pistol configured AR in anticipation of registering the weapon under NFA as an SBR.... build out your AR pistol, pay your $200, get your stamp... NOW you can slap a stock on there. Just remember that once the receiver has been registered under NFA, it will always remain as such, even if you convert it to a non-NFA configuration. That is to say... you can't create an SBR now... then decide you want to sell it, slap a 16" upper onto it, and sell it privately.